Overexpression of glycogen synthase in mouse muscle results in less branched glycogen

Analysis of Cell Glycogen with Quantitation and Determination of Branching Using Liquid Chromatography-Mass Spectrometry

Glycogen is a highly branched biomacromolecule that functions as a glucose buffer. It is involved in multiple diseases such as glycogen storage disorders, diabetes, and even liver cancer, where the imbalance between biosynthetic and catabolic enzymes results in structural alterations and abnormal accumulation of glycogen that can be toxic to cells. Accurate and sensitive glycogen quantification and structural determination are prerequisites for understanding the phenotypes and biological functions of glycogen under these conditions. In this research, we furthered cell glycogen characterization by presenting a highly sensitive method to measure the glycogen content and degree of branching. The method employed a novel fructose density gradient as an alternative to the traditional sucrose gradient to fractionate glycogen from cell mixtures using ultracentrifugation. Fructose was used to avoid the large glucose background, allowing the method to be highly quantitative. The glycogen content was determined by quantifying 1-phenyl-3-methyl-5-pyrazolone (PMP)-derivatized glucose residues obtained from acid-hydrolyzed glycogen using ultra-high-performance liquid chromatography/triple quadrupole mass spectrometry (UHPLC/QqQ-MS). The degree of branching was determined through linkage analysis where the glycogen underwent permethylation, hydrolysis, PMP derivatization, and UHPLC/QqQ-MS analysis. The new approach was used to study the effect of insulin on the glycogen phenotypes of human hepatocellular carcinoma (Hep G2) cells. We observed that cells produced greater amounts of glycogen with less branching under increasing insulin levels before reaching the cell's insulin-resistant state, where the trend reversed and the cells produced less but higher-branched glycogen. The advantage of this method lies in its high sensitivity in characterizing both the glycogen level and the structure of biological samples.

A century of exercise physiology: key concepts in regulation of glycogen metabolism in skeletal muscle

Glycogen is a branched, glucose polymer and the storage form of glucose in cells. Glycogen has traditionally been viewed as a key substrate for muscle ATP production during conditions of high energy demand and considered to be limiting for work capacity and force generation under defined conditions. Glycogenolysis is catalyzed by phosphorylase, while glycogenesis is catalyzed by glycogen synthase. For many years, it was believed that a primer was required for de novo glycogen synthesis and the protein considered responsible for this process was ultimately discovered and named glycogenin. However, the subsequent observation of glycogen storage in the absence of functional glycogenin raises questions about the true role of the protein. In resting muscle, phosphorylase is generally considered to be present in two forms: non-phosphorylated and inactive (phosphorylase b) and phosphorylated and constitutively active (phosphorylase a). Initially, it was believed that activation of phosphorylase during intense muscle contraction was primarily accounted for by phosphorylation of phosphorylase b (activated by increases in AMP) to a, and that glycogen synthesis during recovery from exercise occurred solely through mechanisms controlled by glucose transport and glycogen synthase. However, it now appears that these views require modifications. Moreover, the traditional roles of glycogen in muscle function have been extended in recent years and in some instances, the original concepts have undergone revision. Thus, despite the extensive amount of knowledge accrued during the past 100 years, several critical questions remain regarding the regulation of glycogen metabolism and its role in living muscle.

Glycogen supercompensation is due to increased number, not size, of glycogen particles in human skeletal muscle

NEW FINDINGS

What is the central question of this study? Glycogen supercompensation after glycogen-depleting exercise can be achieved by consuming a carbohydrate-enriched diet, but the associated effects on the size, number and localization of intramuscular glycogen particles are unknown. What is the main finding and its importance? Using transmission electron microscopy to inspect individual glycogen particles visually, we show that glycogen supercompensation is achieved by increasing the number of particles while keeping them at submaximal sizes. This might be a strategy to ensure that glycogen particles can be used fast, because particles that are too large might impair utilization rate.

ABSTRACT

Glycogen supercompensation after glycogen-depleting exercise can be achieved by consuming a carbohydrate-enriched diet, but the associated effects on the size, number and localization of intramuscular glycogen particles are unknown. We investigated how a glycogen-loading protocol affects fibre type-specific glycogen volume density, particle diameter and numerical density in three subcellular pools: between (intermyofibrillar) or within (intramyofibrillar) the myofibrils or beneath the sarcolemma (subsarcolemmal). Resting muscle biopsies from 11 physically active men were analysed using transmission electron microscopy after mixed (MIX), LOW or HIGH carbohydrate consumption separated by glycogen-lowering cycling at 75% of maximal oxygen consumption until exhaustion. After HIGH, the total volumetric glycogen content was 40% [95% confidence interval 16, 68] higher than after MIX in type I fibres (P < 0.001), with little to no difference in type II fibres (9% [95% confidence interval -9, 27]). Median particle diameter was 22.5 (interquartile range 20.8-24.7) nm across glycogen pools and fibre types, and the numerical density was 61% [25, 107] and 40% [9, 80] higher in the subsarcolemmal (P < 0.001) and intermyofibrillar (P < 0.01) pools of type I fibres, respectively, with little to no difference in the intramyofibrillar pool (3% [-20, 32]). In LOW, total glycogen was in the range of 21-23% lower, relative to MIX, in both fibre types, reflected in a 21-46% lower numerical density across pools. In comparison to MIX, particle diameter was unaffected by other diets ([-1.4, 1.3] nm). In conclusion, glycogen supercompensation after prolonged cycling is exclusive to type I fibres, predominantly in the subsarcolemmal pool, and involves an increase in the numerical density rather than the size of existing glycogen particles.

Structure and Regulation of Glycogen Synthase in the Brain

Brain glycogen synthesis is a regulated, multi-step process that begins with glucose transport across the blood brain barrier and culminates with the actions of glycogen synthase and the glycogen branching enzyme to elongate glucose chains and introduce branch points in a growing glycogen molecule. This review focuses on the synthesis of glycogen in the brain, with an emphasis on glycogen synthase, but draws on salient studies in mammalian muscle and liver as well as baker's yeast, with the goal of providing a more comprehensive view of glycogen synthesis and highlighting potential areas for further study in the brain. In addition, deficiencies in the glycogen biosynthetic enzymes which lead to glycogen storage diseases in humans are discussed, highlighting effects on the brain and discussing findings in genetically modified animal models that recapitulate these diseases. Finally, implications of glycogen synthesis in neurodegenerative and other diseases that impact the brain are presented.

A highly prevalent equine glycogen storage disease is explained by constitutive activation of a mutant glycogen synthase

BACKGROUND

Equine type 1 polysaccharide storage myopathy (PSSM1) is associated with a missense mutation (R309H) in the glycogen synthase (GYS1) gene, enhanced glycogen synthase (GS) activity and excessive glycogen and amylopectate inclusions in muscle.

METHODS

Equine muscle biochemical and recombinant enzyme kinetic assays in vitro and homology modelling in silico, were used to investigate the hypothesis that higher GS activity in affected horse muscle is caused by higher GS expression, dysregulation, or constitutive activation via a conformational change.

RESULTS

PSSM1-affected horse muscle had significantly higher glycogen content than control horse muscle despite no difference in GS expression. GS activity was significantly higher in muscle from homozygous mutants than from heterozygote and control horses, in the absence and presence of the allosteric regulator, glucose 6 phosphate (G6P). Muscle from homozygous mutant horses also had significantly increased GS phosphorylation at sites 2+2a and significantly higher AMPKα1 (an upstream kinase) expression than controls, likely reflecting a physiological attempt to reduce GS enzyme activity. Recombinant mutant GS was highly active with a considerably lower K_m for UDP-glucose, in the presence and absence of G6P, when compared to wild type GS, and despite its phosphorylation.

CONCLUSIONS

Elevated activity of the mutant enzyme is associated with ineffective regulation via phosphorylation rendering it constitutively active. Modelling suggested that the mutation disrupts a salt bridge that normally stabilises the basal state, shifting the equilibrium to the enzyme's active state.

GENERAL SIGNIFICANCE

This study explains the gain of function pathogenesis in this highly prevalent polyglucosan myopathy.

A Modified Enzymatic Method for Measurement of Glycogen Content in Glycogen Storage Disease Type IV

Deficiency of glycogen branching enzyme in glycogen storage disease type IV (GSD IV) results in accumulation of less-branched and poorly soluble polysaccharides (polyglucosan bodies) in multiple tissues. Standard enzymatic method, when used to quantify glycogen content in GSD IV tissues, causes significant loss of the polysaccharides during preparation of tissue lysates. We report a modified method including an extra boiling step to dissolve the insoluble glycogen, ultimately preserving the glycogen content in tissue homogenates from GSD IV mice. Muscle tissues from wild-type, GSD II and GSD IV mice and GSD III dogs were homogenized in cold water, and homogenate of each tissue was divided into two parts. One part was immediately clarified by centrifugation at 4°C (STD-prep); the other part was boiled for 5 min then centrifuged (Boil-prep) at room temperature. When glycogen was quantified enzymatically in tissue lysates, no significant differences were found between the STD-prep and the Boil-prep for wild-type, GSD II and GSD III muscles. In contrast, glycogen content for GSD IV muscle in the STD-prep was only 11% of that in the Boil-prep, similar to wild-type values. Similar results were observed in other tissues of GSD IV mice and fibroblast cells from a GSD IV patient. This study provides important information for improving disease diagnosis, monitoring disease progression, and evaluating treatment outcomes in both clinical and preclinical clinical settings for GSD IV. This report should be used as an updated protocol in clinical diagnostic laboratories.

Preclinical Development of New Therapy for Glycogen Storage Diseases

Glycogen storage disease (GSD) consists of more than 10 discrete conditions for which the biochemical and genetic bases have been determined, and new therapies have been under development for several of these conditions. Gene therapy research has generated proof-of-concept for GSD types I (von Gierke disease) and II (Pompe disease). Key features of these gene therapy strategies include the choice of vector and regulatory cassette, and recently adeno-associated virus (AAV) vectors containing tissue-specific promoters have achieved a high degree of efficacy. Efficacy of gene therapy for Pompe disease depend upon the induction of immune tolerance to the therapeutic enzyme. Efficacy of von Gierke disease is transient, waning gradually over the months following vector administration. Small molecule therapies have been evaluated with the goal of improving standard of care therapy or ameliorating the cellular abnormalities associated with specific GSDs. The receptor-mediated uptake of the therapeutic enzyme in Pompe disease was enhanced by administration of β2 agonists. Rapamycin reduced the liver fibrosis observed in GSD III. Further development of gene therapy could provide curative therapy for patients with GSD, if efficacy from preclinical research is observed in future clinical trials and these treatments become clinically available.

Muscle glycogen remodeling and glycogen phosphate metabolism following exhaustive exercise of wild type and laforin knockout mice

Glycogen, the repository of glucose in many cell types, contains small amounts of covalent phosphate, of uncertain function and poorly understood metabolism. Loss-of-function mutations in the laforin gene cause the fatal neurodegenerative disorder, Lafora disease, characterized by increased glycogen phosphorylation and the formation of abnormal deposits of glycogen-like material called Lafora bodies. It is generally accepted that the phosphate is removed by the laforin phosphatase. To study the dynamics of skeletal muscle glycogen phosphorylation in vivo under physiological conditions, mice were subjected to glycogen-depleting exercise and then monitored while they resynthesized glycogen. Depletion of glycogen by exercise was associated with a substantial reduction in total glycogen phosphate and the newly resynthesized glycogen was less branched and less phosphorylated. Branching returned to normal on a time frame of days, whereas phosphorylation remained suppressed over a longer period of time. We observed no change in markers of autophagy. Exercise of 3-month-old laforin knock-out mice caused a similar depletion of glycogen but no loss of glycogen phosphate. Furthermore, remodeling of glycogen to restore the basal branching pattern was delayed in the knock-out animals. From these results, we infer that 1) laforin is responsible for glycogen dephosphorylation during exercise and acts during the cytosolic degradation of glycogen, 2) excess glycogen phosphorylation in the absence of laforin delays the normal remodeling of the branching structure, and 3) the accumulation of glycogen phosphate is a relatively slow process involving multiple cycles of glycogen synthesis-degradation, consistent with the slow onset of the symptoms of Lafora disease.

Laforin is required for the functional activation of malin in endoplasmic reticulum stress resistance in neuronal cells

Mutations in either EPM2A, the gene encoding a dual-specificity phosphatase named laforin, or NHLRC1, the gene encoding an E3 ubiquitin ligase named malin, cause Lafora disease in humans. Lafora disease is a fatal neurological disorder characterized by progressive myoclonus epilepsy, severe neurological deterioration and accumulation of poorly branched glycogen inclusions, called Lafora bodies or polyglucosan bodies, within the cell cytoplasm. The molecular mechanism underlying the neuropathogenesis of Lafora disease remains unknown. Here, we present data demonstrating that in the cells expressing low levels of laforin protein, overexpressed malin and its Lafora disease-causing missense mutants are stably polyubiquitinated. Malin and malin mutants form ubiquitin-positive aggregates in or around the nuclei of the cells in which they are expressed. Neither wild-type malin nor its mutants elicit endoplasmic reticulum stress, although the mutants exaggerate the response to endoplasmic reticulum stress. Overexpressed laforin impairs the polyubiquitination of malin while it recruits malin to polyglucosan bodies. The recruitment and activities of laforin and malin are both required for the polyglucosan body disruption. Consistently, targeted deletion of laforin in brain cells from Epm2a knockout mice increases polyubiquitinated proteins. Knockdown of Epm2a or Nhlrc1 in neuronal Neuro2a cells shows that they cooperate to allow cells to resist ER stress and apoptosis. These results reveal that a functional laforin-malin complex plays a critical role in disrupting Lafora bodies and relieving ER stress, implying that a causative pathogenic mechanism underlies their deficiency in Lafora disease.

Glycogen and its metabolism: some new developments and old themes

Glycogen is a branched polymer of glucose that acts as a store of energy in times of nutritional sufficiency for utilization in times of need. Its metabolism has been the subject of extensive investigation and much is known about its regulation by hormones such as insulin, glucagon and adrenaline (epinephrine). There has been debate over the relative importance of allosteric compared with covalent control of the key biosynthetic enzyme, glycogen synthase, as well as the relative importance of glucose entry into cells compared with glycogen synthase regulation in determining glycogen accumulation. Significant new developments in eukaryotic glycogen metabolism over the last decade or so include: (i) three-dimensional structures of the biosynthetic enzymes glycogenin and glycogen synthase, with associated implications for mechanism and control; (ii) analyses of several genetically engineered mice with altered glycogen metabolism that shed light on the mechanism of control; (iii) greater appreciation of the spatial aspects of glycogen metabolism, including more focus on the lysosomal degradation of glycogen; and (iv) glycogen phosphorylation and advances in the study of Lafora disease, which is emerging as a glycogen storage disease.

Regulation of glycogen metabolism in yeast and bacteria

Microorganisms have the capacity to utilize a variety of nutrients and adapt to continuously changing environmental conditions. Many microorganisms, including yeast and bacteria, accumulate carbon and energy reserves to cope with the starvation conditions temporarily present in the environment. Glycogen biosynthesis is a main strategy for such metabolic storage, and a variety of sensing and signaling mechanisms have evolved in evolutionarily distant species to ensure the production of this homopolysaccharide. At the most fundamental level, the processes of glycogen synthesis and degradation in yeast and bacteria share certain broad similarities. However, the regulation of these processes is sometimes quite distinct, indicating that they have evolved separately to respond optimally to the habitat conditions of each species. This review aims to highlight the mechanisms, both at the transcriptional and at the post-transcriptional level, that regulate glycogen metabolism in yeast and bacteria, focusing on selected areas where the greatest increase in knowledge has occurred during the last few years. In the yeast system, we focus particularly on the various signaling pathways that control the activity of the enzymes of glycogen storage. We also discuss our recent understanding of the important role played by the vacuole in glycogen metabolism. In the case of bacterial glycogen, special emphasis is placed on aspects related to the genetic regulation of glycogen metabolism and its connection with other biological processes.

Restoration of muscle functionality by genetic suppression of glycogen synthesis in a murine model of Pompe disease

Glycogen storage disease type II (GSDII) or Pompe disease is an autosomal recessive disorder caused by acid alpha-glucosidase (GAA) deficiency, leading to lysosomal glycogen accumulation. Affected individuals store glycogen mainly in cardiac and skeletal muscle tissues resulting in fatal hypertrophic cardiomyopathy and respiratory failure in the most severe infantile form. Enzyme replacement therapy has already proved some efficacy, but results remain variable especially in skeletal muscle. Substrate reduction therapy was successfully used to improve the phenotype in several lysosomal storage disorders. We have recently demonstrated that shRNA-mediated reduction of glycogen synthesis led to a significant reduction of glycogen accumulation in skeletal muscle of GSDII mice. In this paper, we analyzed the effect of a complete genetic elimination of glycogen synthesis in the same GSDII model. GAA and glycogen synthase 1 (GYS1) KO mice were inter-crossed to generate a new double-KO model. GAA/GYS1-KO mice exhibited a profound reduction of the amount of glycogen in the heart and skeletal muscles, a significant decrease in lysosomal swelling and autophagic build-up as well as a complete correction of cardiomegaly. In addition, the abnormalities in glucose metabolism and insulin tolerance observed in the GSDII model were corrected in double-KO mice. Muscle atrophy observed in 11-month-old GSDII mice was less pronounced in GAA/GYS1-KO mice, resulting in improved exercise capacity. These data demonstrate that long-term elimination of muscle glycogen synthesis leads to a significant improvement of structural, metabolic and functional defects in GSDII mice and offers a new perspective for the treatment of Pompe disease.

Lafora disease: insights into neurodegeneration from plant metabolism

Reversible phosphorylation modulates nearly every step of glycogenesis and glycogenolysis. Multiple metabolic disorders are the result of defective enzymes that control these phosphorylation events, enzymes that were identified biochemically before the advent of the molecular biology era. Lafora disease is a metabolic disorder resulting in accumulation of water-insoluble glucan in the cytoplasm, and manifests as a debilitating neurodegeneration that ends with the death of the patient. Unlike most metabolic disorders, the link between Lafora disease and metabolism has not been defined in almost 100 years. The results of recent studies with mammalian cells, mouse models, eukaryotic algae, and plants have begun to define the molecular mechanisms that cause Lafora disease. The emerging theme identifies a new phosphorylation substrate in glycogen metabolism, the glucan itself.

Abnormal metabolism of glycogen phosphate as a cause for Lafora disease

Lafora disease is a progressive myoclonus epilepsy with onset in the teenage years followed by neurodegeneration and death within 10 years. A characteristic is the widespread formation of poorly branched, insoluble glycogen-like polymers (polyglucosan) known as Lafora bodies, which accumulate in neurons, muscle, liver, and other tissues. Approximately half of the cases of Lafora disease result from mutations in the EPM2A gene, which encodes laforin, a member of the dual specificity protein phosphatase family that is able to release the small amount of covalent phosphate normally present in glycogen. In studies of Epm2a(-/-) mice that lack laforin, we observed a progressive change in the properties and structure of glycogen that paralleled the formation of Lafora bodies. At three months, glycogen metabolism remained essentially normal, even though the phosphorylation of glycogen has increased 4-fold and causes altered physical properties of the polysaccharide. By 9 months, the glycogen has overaccumulated by 3-fold, has become somewhat more phosphorylated, but, more notably, is now poorly branched, is insoluble in water, and has acquired an abnormal morphology visible by electron microscopy. These glycogen molecules have a tendency to aggregate and can be recovered in the pellet after low speed centrifugation of tissue extracts. The aggregation requires the phosphorylation of glycogen. The aggregrated glycogen sequesters glycogen synthase but not other glycogen metabolizing enzymes. We propose that laforin functions to suppress excessive glycogen phosphorylation and is an essential component of the metabolism of normally structured glycogen.

A role for AGL ubiquitination in the glycogen storage disorders of Lafora and Cori's disease

Cori's disease is a glycogen storage disorder characterized by a deficiency in the glycogen debranching enzyme, amylo-1,6-glucosidase,4-alpha-glucanotransferase (AGL). Here, we demonstrate that the G1448R genetic variant of AGL is unable to bind to glycogen and displays decreased stability that is rescued by proteasomal inhibition. AGL G1448R is more highly ubiquitinated than its wild-type counterpart and forms aggresomes upon proteasome impairment. Furthermore, the E3 ubiquitin ligase Malin interacts with and promotes the ubiquitination of AGL. Malin is known to be mutated in Lafora disease, an autosomal recessive disorder clinically characterized by the accumulation of polyglucosan bodies resembling poorly branched glycogen. Transfection studies in HepG2 cells demonstrate that AGL is cytoplasmic whereas Malin is predominately nuclear. However, after depletion of glycogen stores for 4 h, approximately 90% of transfected cells exhibit partial nuclear staining for AGL. Furthermore, stimulation of cells with agents that elevate cAMP increases Malin levels and Malin/AGL complex formation. Refeeding mice for 2 h after an overnight fast causes a reduction in hepatic AGL levels by 48%. Taken together, these results indicate that binding to glycogen crucially regulates the stability of AGL and, further, that its ubiquitination may play an important role in the pathophysiology of both Lafora and Cori's disease.

Laforin is a glycogen phosphatase, deficiency of which leads to elevated phosphorylation of glycogen in vivo

Lafora disease is a progressive myoclonus epilepsy with onset typically in the second decade of life and death within 10 years. Lafora bodies, deposits of abnormally branched, insoluble glycogen-like polymers, form in neurons, muscle, liver, and other tissues. Approximately half of the cases of Lafora disease result from mutations in the EPM2A gene, which encodes laforin, a member of the dual-specificity protein phosphatase family that additionally contains a glycogen binding domain. The molecular basis for the formation of Lafora bodies is completely unknown. Glycogen, a branched polymer of glucose, contains a small amount of covalently linked phosphate whose origin and function are obscure. We report here that recombinant laforin is able to release this phosphate in vitro, in a time-dependent reaction with an apparent K(m) for glycogen of 4.5 mg/ml. Mutations of laforin that disable the glycogen binding domain also eliminate its ability to dephosphorylate glycogen. We have also analyzed glycogen from a mouse model of Lafora disease, Epm2a(-/-) mice, which develop Lafora bodies in several tissues. Glycogen isolated from these mice had a 40% increase in the covalent phosphate content in liver and a 4-fold elevation in muscle. We propose that excessive phosphorylation of glycogen leads to aberrant branching and Lafora body formation. This study provides a molecular link between an observed biochemical property of laforin and the phenotype of a mouse model of Lafora disease. The results also have important implications for glycogen metabolism generally.

Glycogen metabolism in tissues from a mouse model of Lafora disease

Laforin, encoded by the EPM2A gene, by sequence is a member of the dual specificity protein phosphatase family. Mutations in the EPM2A gene account for around half of the cases of Lafora disease, an autosomal recessive neurodegenerative disorder, characterized by progressive myoclonus epilepsy. The hallmark of the disease is the presence of Lafora bodies, which contain polyglucosan, a poorly branched form of glycogen, in neurons, muscle and other tissues. Glycogen metabolizing enzymes were analyzed in a transgenic mouse over-expressing a dominant negative form of laforin that accumulates Lafora bodies in several tissues. Skeletal muscle glycogen was increased 2-fold as was the total glycogen synthase protein. However, the -/+glucose-6-P activity of glycogen synthase was decreased from 0.29 to 0.16. Branching enzyme activity was increased by 30%. Glycogen phosphorylase activity was unchanged. In whole brain, no differences in glycogen synthase or branching enzyme activities were found. Although there were significant differences in enzyme activities in muscle, the results do not support the hypothesis that Lafora body formation is caused by a major change in the balance between glycogen elongation and branching activities.

Proglycogen, macroglycogen, glucose, and glucose-6-phosphate concentrations in skeletal muscles of horses with polysaccharide storage myopathy performing light exercise

OBJECTIVE

To determine concentrations of proglycogen (PG), macroglycogen (MG), glucose, and glucose-6-phosphate (G-6-P) in skeletal muscle of horses with polysaccharide storage myopathy (PSSM) before and after performing light submaximal exercise.

ANIMALS

6 horses with PSSM and 4 control horses.

PROCEDURES

Horses with PSSM completed repeated intervals of 2 minutes of walking followed by 2 minutes of trotting on a treadmill until muscle cramping developed. Four untrained control horses performed a similar exercise test for up to 20 minutes. Serum creatine kinase (CK) activity was measured before and 4 hours after exercise. Concentrations of total glycogen (G(t)), PG, MG, G-6-P, free glucose, and lactate were measured in biopsy specimens of gluteal muscle obtained before and after exercise.

RESULTS

Mean serum CK activity was 26 times higher in PSSM horses than in control horses after exercise. Before exercise, muscle glycogen concentrations were 1.5, 2.2, and 1.7 times higher for PG, MG, and G(t), respectively, in PSSM horses, compared with concentrations in control horses. No significant changes in G(t), PG, MG, G-6-P, and lactate concentrations were detected after exercise. However, free glucose concentrations in skeletal muscle increased significantly in PSSM horses after exercise.

CONCLUSIONS AND CLINICAL RELEVANCE

Analysis of the results suggests that glucose uptake in skeletal muscle is augmented in horses with PSSM after light exercise. There is excessive storage of PG and MG in horses with PSSM, and high concentrations of the 2 glycogen fractions may affect functional interactions between glycogenolytic and glycogen synthetic enzymes and glycosomes.

Laforin, a dual specificity phosphatase that dephosphorylates complex carbohydrates

Laforin is the only phosphatase in the animal kingdom that contains a carbohydrate-binding module. Mutations in the gene encoding laforin result in Lafora disease, a fatal autosomal recessive neurodegenerative disorder, which is diagnosed by the presence of intracellular deposits of insoluble complex carbohydrates known as Lafora bodies. We demonstrate that laforin interacts with proteins known to be involved in glycogen metabolism and rule out several of these proteins as potential substrates. Surprisingly, we find that laforin displays robust phosphatase activity against a phosphorylated complex carbohydrate. Furthermore, this activity is unique to laforin, since several other phosphatases are unable to dephosphorylate polysaccharides. Finally, fusing the carbohydrate-binding module of laforin to the dual specific phosphatase VHR does not result in the ability of this phosphatase to dephosphorylate polysaccharides. Therefore, we hypothesize that laforin is unique in its ability to utilize a phosphorylated complex carbohydrate as a substrate and that this function may be necessary for the maintenance of normal cellular glycogen.

Genome-wide linkage analyses of type 2 diabetes in Mexican Americans: the San Antonio Family Diabetes/Gallbladder Study

The San Antonio Family Diabetes/Gallbladder Study was initiated to identify susceptibility genes for type 2 diabetes. Evidence was previously reported of linkage to diabetes on 10q with suggestive evidence on 3p and 9p in a genome-wide scan of 440 individuals from 27 pedigrees ascertained through a single diabetic proband. Subsequently, the study was expanded to include 906 individuals from 39 extended Mexican-American pedigrees, two additional examination cycles approximately 5.3 and 7.6 years after baseline, and genotypes for a new set of genome-wide markers. Therefore, we completed a second genome-wide linkage scan. Using information from a participant's most recent exam, the prevalence of diabetes in nonprobands was 21.8%. We performed genome-wide variance components-based genetic analysis on the discrete trait diabetes using a liability model and on the quantitative Martingale residual obtained from modeling age of diabetes diagnosis using Cox proportional hazard models. Controlling for age and age(2), our strongest evidence for linkage to the trait diabetes and the quantitative Martingale residual was on chromosome 3p at marker D3S2406 with multipoint empirical logarithm of odds scores of 1.87 and 3.76, respectively. In summary, we report evidence for linkage to diabetes on chromosome 3p in a region previously identified in at least three independent populations.

Mice with elevated muscle glycogen stores do not have improved exercise performance

Skeletal muscle glycogen is considered to be an important source of energy for contraction and increasing the level of the glucose polymer is generally thought to improve exercise performance in humans. A genetically modified mouse model (GSL30), which overaccumulates glycogen due to overexpression of a hyperactive form of glycogen synthase, was used to examine whether increasing the level of the polysaccharide enhances the ability of mice to run on a treadmill. The skeletal muscle of the GSL30 mice had large deposits of glycogen. There were no significant increases in the work performed by GSL30 mice as compared to their respective wild type littermates when exercised to exhaustion. The amount of muscle glycogen utilized by GSL30 mice, however, was greater, while the amount of liver glycogen consumed during exhaustive exercise was less than wild type animals. This result suggests that increased muscle glycogen stores do not necessarily improve exercise performance in mice.

Yeast glycogenin (Glg2p) produced in Escherichia coli is simultaneously glucosylated at two vicinal tyrosine residues but results in a reduced bacterial glycogen accumulation

Saccharomyces cerevisiae possesses two glycogenin isoforms (designated as Glg1p and Glg2p) that both contain a conserved tyrosine residue, Tyr232. However, Glg2p possesses an additional tyrosine residue, Tyr230 and therefore two potential autoglucosylation sites. Glucosylation of Glg2p was studied using both matrix-assisted laser desorption ionization and electrospray quadrupole time of flight mass spectrometry. Glg2p, carrying a C-terminal (His6) tag, was produced in Escherichia coli and purified. By tryptic digestion and reversed phase chromatography a peptide (residues 219-246 of the complete Glg2p sequence) was isolated that contained 4-25 glucosyl residues. Following incubation of Glg2p with UDPglucose, more than 36 glucosyl residues were covalently bound to this peptide. Using a combination of cyanogen bromide cleavage of the protein backbone, enzymatic hydrolysis of glycosidic bonds and reversed phase chromatography, mono- and diglucosylated peptides having the sequence PNYGYQSSPAM were generated. MS/MS spectra revealed that glucosyl residues were attached to both Tyr232 and Tyr230 within the same peptide. The formation of the highly glucosylated eukaryotic Glg2p did not favour the bacterial glycogen accumulation. Under various experimental conditions Glg2p-producing cells accumulated approximately 30% less glycogen than a control transformed with a Glg2p lacking plasmid. The size distribution of the glycogen and extractable activities of several glycogen-related enzymes were essentially unchanged. As revealed by high performance anion exchange chromatography, the intracellular maltooligosaccharide pattern of the bacterial cells expressing the functional eukaryotic transgene was significantly altered. Thus, the eukaryotic glycogenin appears to be incompatible with the bacterial initiation of glycogen biosynthesis.

Increased glycogen storage in yeast results in less branched glycogen

Glycogen is a branched polymer of glucose, synthesized as a reserve of both energy and carbon. The branched nature of glycogen is important for its function and polyglucosan bodies, particles that contain a glycogen-like polymer with reduced branching, are a feature of several disease states. The degree of glycogen branching is thought to be governed by the balance between glycogen synthesis and branching activities. However, there have been reports that the intrinsic properties of individual branching enzymes govern the degree of branching. To address the relationship between synthesis and branching more fully, we made use of the yeast Saccharomyces cerevisiae. The glycogen content of yeast cells was manipulated by using different growth conditions or by the introduction of specific mutations. Whenever glycogen storage was elevated, the polysaccharide formed was found to be less branched but normal branching could be restored by overexpression of branching enzyme.